Speaker excursion prediction and protection

ABSTRACT

Systems and devices configured by an algorithm to prevent or limit a speaker over-excursion are disclosed. The disclosed algorithm is computationally efficient because it exploits a relationship between an audio signal and a speaker&#39;s excursion that exists at low-frequencies, below a self-resonance of a speaker. The disclosed algorithm combines the low-frequency excursion protection with a high-frequency, transient excursion protection. The combined approach allows the transient excursion protection to use a shorter delay than otherwise possible. The shorter delay allows for a compressor to apply attenuation to a transient audio signal before a momentum of the speaker, caused by the transient audio signal, grows too large to be controlled.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.16/785,968 filed on Feb. 10, 2020, which is hereby incorporated byreference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to audio systems and more specifically tosystems and methods for protecting a speaker from excessive movement(i.e., over excursion).

BACKGROUND

Audio speakers (i.e., speakers) are designed to operate below a maximumexcursion. It may be possible (e.g., through amplification or signalprocessing) to drive a speaker to move in excess of its maximumexcursion. This excess movement can result in damage to, or destructionof, the speaker. For example, a coil of a speaker may be driven to anunguided position, at which point it may become permanently lodged.Additionally, a suspension which supports a speaker cone may becomepermanently stretched so that sounds from the speaker are distorted. Theexcess movement can also result in a degradation of a sound quality orunwanted sounds from the speaker. Accordingly, it may be desirable toprotect a speaker from over excursion. It is in this context thatimplementations of the disclosure arise.

SUMMARY

In at least one aspect, the present disclosure generally describes acomputing device. The computing device includes a speaker that isconfigured to move according to an audio signal. The computing devicealso includes a processor that is configured by software instructions toperform a method to limit movements of the speaker to below a maximumexcursion. The method includes receiving the audio signal. The methodfurther includes applying a low-frequency excursion protection to theaudio signal. The low-frequency excursion protection includesdetermining a sub-resonance excursion prediction of a movement of thespeaker, comparing the sub-resonance excursion prediction to the maximumexcursion, and adjusting the audio signal when the sub-resonanceexcursion prediction exceeds the maximum excursion. After thelow-frequency excursion protection is applied, a transient excursionprotection is applied to the audio signal. The transient excursionprotection includes determining an instantaneous excursion prediction ofthe movement of the speaker, comparing the instantaneous excursionprediction to the maximum excursion, and adjusting the audio signal whenthe instantaneous excursion prediction exceeds the maximum excursion.

In at least one other aspect, the present disclosure generally describesa method to limit a movement of a speaker to below a maximum excursion.The method includes receiving an audio signal. The method furtherincludes applying a low-frequency protection to the audio signal, whichincludes determining a sub-resonance excursion prediction of themovement of the speaker, comparing the sub-resonance excursionprediction to the maximum excursion, and adjusting the audio signal whenthe sub-resonance excursion prediction exceeds the maximum excursion.After applying the low-frequency excursion protection of the audiosignal, the method includes applying a transient excursion protection tothe audio signal. The transient excursion protection includesdetermining an instantaneous excursion prediction of the movement of thespeaker, comparing the instantaneous excursion prediction to the maximumexcursion, and adjusting the audio signal when the instantaneousexcursion prediction exceeds the maximum excursion.

In at least one other aspect, the present disclosure generally describesa smart amplifier. The smart amplifier includes a processor that isconfigured by software instructions to perform a method. The methodincludes receiving an audio signal and receiving a sensed signal. Thesensed signal corresponds to a response (i.e., an audio signal response)of a speaker that is coupled to the smart amplifier. The method furtherincludes deriving speaker parameters from the sensed signal and updatingat least one speaker model based on the derived speaker parameters. Themethod further includes applying a low-frequency excursion protection tothe audio signal. The low-frequency excursion protection includesdetermining a sub-resonance excursion prediction of a movement of thespeaker that is based on the updated at least one speaker model,comparing the sub-resonance excursion prediction to a maximum excursion,and adjusting the audio signal when the sub-resonance excursionprediction exceeds the maximum excursion to limit movements of thespeaker to below a maximum excursion. After applying the low-frequencyexcursion protection, the method includes applying a transient excursionprotection to the audio signal. The transient excursion protectionincludes determining an instantaneous prediction of the movement of thespeaker that is based on the updated at least one speaker model,comparing the instantaneous prediction to the maximum excursion andadjusting the audio signal when the instantaneous prediction exceeds themaximum excursion to limit movements of the speaker to below the maximumexcursion.

The foregoing illustrative summary, as well as other exemplaryobjectives and/or advantages of the disclosure, and the manner in whichthe same are accomplished, are further explained within the followingdetailed description and its accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a computing device configured foraudio playback with excursion protection according to a possibleimplementation of the present disclosure.

FIG. 2 illustrates a block diagram of an audio system with excursionprotection according to a possible implementation of the presentdisclosure.

FIG. 3 illustrates a block diagram of a smart amplifier configured toamplify and audio signal with excursion protection according to apossible implementation of the present disclosure.

FIG. 4 illustrates a block diagram of an excursion protection moduleaccording to a possible implementation of the present disclosure.

FIG. 5 illustrates a block diagram of a low-frequency excursionprotection component of the excursion protection implementation shown inFIG. 4.

FIG. 6 illustrates a block diagram of a transient excursion protectioncomponent of the excursion protection implementation shown in FIG. 4.

FIG. 7 is a flow chart of a method to limit a movement of a speaker tobelow a maximum excursion according to a possible implementation of thepresent disclosure.

The components in the drawings are not necessarily to scale relative toeach other. Like reference numerals designate corresponding partsthroughout the several views.

DETAILED DESCRIPTION

The present disclosure describes systems and methods for adaptivelyadjusting an audio signal to prevent damage to or distortion of soundsfrom a speaker as a result of excessive (i.e. over) excursion. Thedisclosed approach combines low-frequency excursion protection withtransient excursion protection (e.g., high-frequency excursionprotection). For the low-frequency excursion protection, the approachutilizes a simplification for determining an excursion based on ananalysis of sub-resonance frequencies of an audio signal. The disclosedsystems and methods may provide the advantageous results of excursionprotection without significant audio distortion and with lowcomputational complexity (i.e., high computational efficiency). The lowcomputational complexity can make the disclosed approach especiallyuseful in mobile computing devices (e.g., mobile phones, tabletcomputers), which are typically battery operated and which tend to havesmall speakers that can easily be overdriven into excessive excursion.Additionally, the low-frequency excursion protection and the transientexcursion protection combine in a way that can limit a delay caused bythe excursion protection. Accordingly, the disclosed systems and methodsmay not significantly affect an audio signal that does not requireexcursion protection.

The disclosed circuits and methods may be used in a variety of systemsincluding but not limited to smart speakers (e.g., voice-activatedvirtual assistant), mobile devices (e.g., mobile phones, tablets), audioprocessors (e.g., digital mixing board), and smart amplifiers (e.g.,integrated amplifier circuit). The disclosed circuits and methods may beimplemented in a digital portion of an audio processing stream beforethe audio is converted to an analog audio signal and transmitted to aspeaker.

FIG. 1 illustrates a block diagram of a computing device configured foraudio playback with excursion protection according to a possibleimplementation of the present disclosure. The computing device 100includes a speaker 110. The speaker may be of a variety of types andmaterials that are configured to convert an electrical, analog audiosignal into a corresponding sound wave 115. The speaker 110 may includea voice coil (i.e., coil) positioned within a magnetic field created bya magnet so that a current in the coil and the magnetic field canproduce a physical force that can move (displace) a diaphragm (e.g.,cone) of the speaker by an excursion amount determined by the audiosignal. The speaker includes a suspension that supports the diaphragmand provides returning force to counter an excursion 120 (i.e.,displacement) of the speaker 110.

The computing device 100 may also include an amplifier 125 (e.g., poweramplifier) and a digital-to-analog (D/A) converter 130 that togethertransform a digital audio signal to an analog audio signal at anappropriate level to drive the speaker. When the speaker is driven withan audio signal having an amplitude above an appropriate level (i.e.,when the speaker is over-driven), however, an excursion 120 of thespeaker 110 may exceed a maximum excursion specified for the speaker 110and damage or change to the speaker 110 may result.

A speaker can be over-driven beyond its designed maximum excursion(i.e., excursion limit) by a variety of sources. Some sources areintentional. For example, an audio signal may be processed to enhance abass response, thereby creating low-frequency components which arebeyond a capability of a speaker (e.g., a small, multipurpose speaker).Some sources are unintentional. For example, a transient signal maycreate a high-amplitude, short-duration (i.e., impulse) signal that canmove a speaker more than, for example, semi-steady tones created bymusic or speech. The movement resulting from over-driving the speakermay exceed a maximum excursion by an over-excursion amount (i.e., anover-excursion). Excursion protection of the speaker may require anadjustment to the audio signal that is in proportion to theover-excursion.

In some implementations, the computing device may further include asensor 135 proximate to, or integrated with, the speaker. In onepossible implementation, the sensor 135 can be configured to generate asensed signal corresponding to a position and/or a movement (e.g.,velocity, acceleration) of the speaker. In another possibleimplementation, the sensor 135 can be configured to generate a sensedsignal corresponding to a voltage (e.g., a driving voltage) or a current(e.g., driving current) of the speaker. The voltage/current measurementscan be digitized and fed back to the processor. The sensed signal may beused to determine (i.e., derive) speaker parameters that describe (i.e.model) the operation of the speaker. Accordingly, in someimplementations it is possible to update (e.g., periodically update) aspeaker model to account for physical changes to the environment and/orthe speaker. Possible speaker parameters than can be used to model thespeaker are shown in TABLE 1.

TABLE 1 EXAMPLE SPEAKER PARAMETERS SPEAKER PARAMETER SYMBOL UNITDiaphragm Mass M_(ms) g Suspension Stiffness K_(ms) N/m MechanicalResistance of Suspension R_(ms) N · s/m Direct Current (DC) Resistanceof Coil R_(e) Ω Force Factor of Magnetic Field on Coil B1 T · m ResonantFrequency F_(s) Hz

The computing device 100 may also include a memory 140. For example, thecomputing device may include a non-transitory computer readable memory,which stores computer-readable instructions that when executed by aprocessor 145 can cause the processor (and the computing device moregenerally) to perform a method, such as a method for excursionprotection 150 or a method for interfacing with an audio source (i.e.,audio interface 155).

In some implementations, the audio interface 155 may be configured toreceive or retrieve audio data from an audio source (e.g., a file, a bitstream, a device, etc.). For example, the audio interface 155 mayretrieve audio data from the memory 140 or from a network 160, whichwired or wirelessly, can couple audio sources to the computing device100. The audio interface 155 may further translate the received audiointo a digital audio signal (i.e., audio signal).

The memory may also include a speaker model that describes the operationof the speaker. For example, the speaker model may define a displacement(X) of the speaker based on an input audio signal using, for example,one or more of the speaker parameters listed in TABLE 1. The memory mayalso include audio files, which can be stored in a variety of digitalaudio formats. In a possible implementation, the audio interface 155 mayretrieve an audio file from the memory and transform the audio file intothe audio signal.

The memory may also include software instructions, modules, programs andthe like. For example, the memory may include computer-readableinstructions that when executed by the processor perform a method forlimiting a movement of a speaker to below a maximum excursion. Whenconfigured, the processor can analyze aspects of the audio signal topredict an excursion of the speaker and adjust the audio signal when thepredicted excursion is determined to exceed a (predetermined) maximumexcursion. The disclosed approach can provide this analysis/adjustmentwithout significantly affecting (e.g., delaying) an audio data streamand so it can be combined with another forms of excursion protectionand/or audio processing.

The computing device configured for audio playback with excursionprotection shown in FIG. 1 is completely integrated, including bothdigital and analog portions of audio playback. The disclosed approachmay be implemented in a variety of other hardware configurations, suchas shown in FIGS. 2 and 3. The various implementations shown in thesefigures represent a range of possible implementations, which can includevarious combinations and/or sub-combinations of the functions,components and/or features of the different implementations. Forexample, FIG. 2 illustrates a block diagram of an audio system withexcursion protection according to a possible implementation of thepresent disclosure. The system 200 includes a computing device 210, anamplifier 220 and a speaker 110 that are physically distinct butcommunicatively coupled. In a possible implementation the system 200 mayinclude a plurality of speakers.

In another example, FIG. 3 illustrates a block diagram of a smartamplifier configured to amplify and audio signal with excursionprotection according to a possible implementation of the presentdisclosure. The smart amplifier 300 is configured to couple an audiosource 320 to a speaker 110. In some implementations, the smartamplifier can be configured to receive (e.g., via an analog to digitalconverter) a sensed signal from the speaker. The sensed signal cancorrespond to a response of the speaker to the audio signal (e.g., aposition or a movement) and/or a signal associated with the speaker(e.g., a voltage or a current). A processor 325 (e.g., a digital signalprocessor (DSP)) of the smart amplifier 300 can be configured to derivespeaker parameters (e.g., as shown in TABLE 1) from the sensed signal.In a possible implementation, the derived speaker parameters may be usedto update a speaker model.

A speaker model can be implemented as a mathematical relationship (e.g.,equation) between a position/displacement of the speaker (X) and aninput audio signal (e.g., V_(IN)). In other words, a speaker model maybe used to predict a position (i.e., excursion) of the speaker for agiven input audio signal (i.e. audio signal).

A speaker responds to an audio signal differently based on the audiosignal's frequency content (i.e., spectrum). Like a mass and spring, thespeaker may exhibit a resonant frequency (i.e., resonance,self-resonance) at which it most readily responds to an input.Accordingly, the speaker may respond to input signals at frequenciesbelow the resonant frequency (F_(s)) of the speaker differently thaninput signals at frequencies above the resonant frequency of thespeaker.

For an input audio signal below the resonant frequency, the speakermoves in phase and in proportion with the input audio signal (i.e.V_(IN)), thereby simplifying the speaker model for sub-resonance (i.e.,sub-resonant) frequencies, as shown below.X(n)=C·V _(in)(n)  (1)

For equation (1) above, X is speaker displacement, V_(in) is audiosignal, n is a sample, and C is a constant of proportionality. In someimplementations, C may be described by speaker parameters, as shownbelow.

$\begin{matrix}{C = \frac{Bl^{2}}{R_{e} \cdot K_{ms}}} & (2)\end{matrix}$

Thus, for low-frequency audio signals predicting an excursion may bemade simple by the speaker model for sub-resonance frequency audiosignals. Accordingly, the disclosed approach, utilizes thesimplification in a sub-resonance excursion prediction to applylow-frequency excursion protection in a computationally efficientmanner. This approach may be advantageous because over-excursion may beeasily caused by sub-resonance (e.g., bass) frequencies.

Over-excursion may also be easily caused by transient signals (e.g.,pops, static, etc.) and low-frequency excursion protection may fail topredict and adjust for these transient signals. Accordingly, thedisclosed approach combines a low-frequency excursion protection with atransient excursion protection.

FIG. 4 illustrates a block diagram of an excursion protection module(i.e., excursion protection) according to a possible implementation ofthe present disclosure. The excursion protection 150 includes alow-frequency excursion protection 500 component and a transientexcursion protection 600 component. The excursion protection 150 isconfigured to receive an input audio signal 410 (i.e. audio signal 410)at an input to the low-frequency excursion protection 500. Thelow-frequency excursion protection 500 is configured to adjust the audiosignal 410 and return at its output a high-pass filtered (HPF) versionof the audio signal (i.e., HPF audio signal 420). The HPF audio signal420 is then input to the transient excursion protection 600. Thetransient excursion protection 600 is configured to adjust the HPF audiosignal and return, at its output, an audio signal that protects thespeaker from over excursion (i.e., a protected audio signal 430). Insome implementations, the excursion protection may be configured toreceive a sensed signal 440 (e.g., from a speaker sensor) that can beused, for example, to update the low-frequency excursion protection 500and/or the transient excursion protection 600 to changes in the speaker.

FIG. 5 illustrates a block diagram of a low-frequency excursionprotection component of the excursion protection implementation shown inFIG. 4. The low-frequency excursion protection 500 includes a fixedhigh-pass filter 510. The fixed high-pass filter (HPF) is configured topass frequencies of the audio signal 410 that are above a fixed-cutofffrequency (F_(c_fix)). In a possible implementation, the fixed-cutofffrequency is below the resonant frequency of the speaker (Fs). Forexample, the fixed-cutoff frequency may be ¼ the resonance frequency ofthe speaker, such as shown below.

$\begin{matrix}{F_{c\;\_\;{fix}} = {\frac{1}{4} \cdot F_{s}}} & (3)\end{matrix}$

The frequencies blocked by the fixed high-pass filter 510 may haveminimal effect on the protected audio signal 420 because the speaker mayhave very little (if any) sensitivity for (i.e., response to) theblocked frequency components.

The low-frequency excursion protection 500 further includes anadjustable-high-pass filter 550. The adjustable-high-pass filter 550 isconfigured to pass frequencies of a filtered audio signal 412 above anadjustable-cutoff frequency (F_(c_adj)). In a normal (i.e., default)state (i.e., when exclusion protection is unnecessary), theadjustable-cutoff frequency may be equal to the fixed-cutoff frequencyso that the HPF audio signal 420 is perceived as approximatelyequivalent to the audio signal 410 when played on the speaker 110. Forexample, a speaker may have a similar (e.g., the same) response to theHPF audio signal 420 and the audio signal 410 whenF_(c_fix)=F_(c_adj)=1/4F_(s).

Low-frequency excursion protection may be accomplished by adjusting(e.g., raising) the adjustable-cutoff frequency so that additional lowfrequency components (above 1/4F_(s)) of the audio signal are blocked.In a possible implementation, the adjustable-cutoff frequency isadjustable over the range shown below.

$\begin{matrix}{F_{c\;\_\;{adj}} \geq {\frac{1}{4} \cdot F_{s}}} & (4)\end{matrix}$

The amount of adjustment can depend on a prediction of an excursion thatresults from sub-resonance frequencies of the audio signal 410 thatcause a significant response in the speaker.

The low-frequency excursion protection 500 includes a low-pass filter520 (LPF). The low-pass filter is configured to pass frequencies of theaudio signal 410 that are below a low-pass-cutoff frequency (F_(c_lp)).In other words, the low-pas filter passes a low-pass filtered version ofthe audio signal. In a possible implementation, the low-pass-cutofffrequency is the resonant frequency of the speaker (F_(s)), such asshown below.F _(c_lp) =F _(s)  (5)

After passing through the fixed high-pass filter 510 and the low-passfilter 520, the audio signal can contain only frequency components thatare at and below the resonance frequency of the speaker. As describedabove, for audio signals in this frequency regime, the speaker excursioncan be predicted by a speaker model 545 in which the excursion isproportional to the audio signal (e.g., see Equation (1)), and in whichthe speaker parameters of the model are pre-measured (e.g., factory set)or measured on-line (e.g., from a sensor). Accordingly, thelow-frequency excursion protection 500 includes peak detection 530 whichis applied to the speaker model 545 to predict an excursion (i.e., amovement) of the speaker due to sub-resonance frequencies (i.e.,sub-resonance excursion prediction 540). The sub-resonance excursionprediction may be further configured compare the predicted excursion toa threshold (e.g., a maximum excursion). When the sub-resonanceexcursion prediction of the movement of the speaker exceeds a maximumexcursion, the adjustable cutoff-frequency of the adjustable-high-passfilter 550 may be raised. When raised, the adjustable-high-pass filter550 may block the frequencies of the audio signal that were predicted tocause and over excursion. In one possible implementation thesub-resonance excursion prediction 540 computes a difference between thesub-resonance excursion prediction and the maximum excursion as alow-frequency over-excursion. The sub-resonance excursion prediction 540can then be configured to generate a signal that raises the adjustablecut-off frequency of the adjustable-high-pass filter 550. Variousalgorithms can be used to determine a relationship between thelow-frequency over-excursion and the amount the adjustable cut-offfrequency is raised. In one possible implementation, the adjustablecut-off frequency is raised in proportion to the low-frequencyover-excursion.

Because transients in the audio signal can cause a speaker to move morethan a steady-state tone, the low-frequency excursion protection 500 maybe insufficient in eliminating or reducing all over-excursion signalsfrom the audio signal. Accordingly, the HPF version of the audio signalat the output of the low-frequency excursion protection is protectedagainst over-excursions due to transients.

FIG. 6 illustrates a block diagram of a transient excursion protectioncomponent of the excursion protection implementation shown in FIG. 4.The transient excursion protection 600 includes an amplifier 640 with anadjustable gain. When a transient audio signal is predicted in thereceived HPF audio signal 420, the adjustable gain of the amplifier 640can be reduced to decrease and/or limit an excursion caused by thetransient audio signal. In other words, the amplifier 640 can operate asa variable attenuator that can attenuate audio signals having atransient.

To prevent a transient audio signal from causing a movement in thespeaker with inertia that cannot be slowed or stopped by a change to theadjustable gain, the transient excursion protection includes a delaybuffer (i.e., delay 610). The delay provides the transient excursionprotection 600 time to adjust the gain of the amplifier 640 before themovement of the speaker reaches an inertia that cannot be overcome witha simple gain adjustment. An advantageous aspect of the disclosedapproach is that the delay is not large because the transient excursionprotection 600 receives a high-pass filtered version of the audio signal420. In other words, the size of the delay can correspond to the lowestfrequencies of the audio signal received at the input of the transientexcursion protection. Because this signal audio signal is high-passedfiltered by the previous protection module, the delay can be reduced.Thus, in the disclosed approach the low-frequency excursion protection500 facilitates a transient excursion protection having a small delay.When the size of the delay 610 is made small, the impact (i.e., affect)of the excursion protection 150 on the digital audio stream that itmonitors and adjusts is reduced, which can be advantageous for a varietyof reasons including (but not limited to) an invisibility to a user, acompatibility with other audio processing/devices, and the like. Whilethe delay can be made small, it may be sufficiently large so as not tocreate an audible artifact.

The transient excursion protection 600 includes an instantaneousexcursion prediction 620 based on a speaker model 625, in which thespeaker parameters of the model are pre-measured (e.g., factory set) ormeasured on-line (e.g., from a sensor). A variety of speaker models maybe implemented, and the disclosed approach is not limited to anyparticular speaker model. In one possible implementation, the speakermodel for the instantaneous excursion prediction may be as shown below.X(n)=d·(a·V(n)+b·X(n−1)+c·X(n−2))  (6)

For equation (6) above, X is speaker displacement, V is the HPF audiosignal, n is the current (instantaneous) sample, n−1 is a previoussample, and n−2 is a previous sample. In other words, the instantaneousexcursion prediction of the movement of the speaker may be based on theaudio signal and previous positions of the speaker. The second-orderequation includes constants a, b, c, and d described by speakerparameters, as shown below, where T_(s) is the sample period.

$\begin{matrix}{a = {T_{s}^{2} \cdot \frac{Bl}{R_{e}}}} & (7) \\{b = {{R_{ms} \cdot T_{s}} + {2M_{ms}}}} & (8) \\{c = {- M_{ms}}} & (9) \\{d = {1/\left( {{K_{ms} \cdot T_{s}^{2}} + {R_{ms} \cdot T_{s}} + M_{ms}} \right)}} & (10)\end{matrix}$

In a possible implementation, the constants, a, b, c, and d can beupdated (e.g., periodically, as necessary) based on a sensed signal 440from the speaker.

A compressor 630 is configured to receive a predicted excursion from theinstantaneous excursion prediction 620. The compressor can compare thepredicted excursion to a threshold (i.e., a limit). For example, if thepredicted excursion is above a (predetermined) maximum excursion thenthe compressor 630 may generate a signal to control the gain of theadjustable gain amplifier 640 to attenuate the signal out of the delay610 (i.e., the delayed high-pass filtered version). The amount ofattenuation (i.e., a compression ratio) can increase proportionally witha predicted over-excursion. For example, when a predicted over-excursionis 3 decibels (dB) above a maximum excursion, the attenuation appliedmay be 3 dB. Because the attenuation occurs after the delay 610, aportion of the audio signal prior to the transient is attenuated. Thisattenuation of the audio signal before the transient reduces a momentumof the speaker prior to the transient. This reduction in momentum helpsprevent the transient from adding enough momentum to create anuncontrollable movement.

A maximum excursion (i.e., excursion limit) that is used to triggerexcursion protection may have a value that is determined variously. Forexample, a maximum excursion used for filter adjustment and/orcompression adjustment may be derived/measured for a class of speakersor may be derived/measured on a speaker-by-speaker basis. The derivationand/or measurement of the maximum excursion may be performed at a timebefore implementation (e.g., factory set). In a possible implementation,the maximum excursion may be adjustable (e.g., customer controllable).The adjustability may provide more versatility in protection (e.g.,derating) for different classes of expected audio signals.

FIG. 7 is a flow chart of a method to limit a movement of a speaker tobelow a maximum excursion according to a possible implementation of thepresent disclosure. The method 700 includes receiving 710 an audiosignal. To limit a movement of a speaker to below a maximum excursion,the method 700 creates a protected audio signal 750 that is analyzed forits potential in driving a speaker to an over excursion and adjusted asnecessary to prevent the over excursion. The method first applies 720 alow-frequency excursion protection. The low-frequency excursionprotection includes determining 721 a sub-resonance excursionprediction, which can correspond to an amount of excursion of thespeaker based on the frequency components of the audio signal below aresonance of the speaker. The sub-resonance excursion prediction is thencompared 722 to a maximum excursion. When the prediction exceeds 723 (Y)the maximum excursion (or otherwise satisfies an excursion condition),the audio signal is adjusted (e.g., filtered to remove frequencycomponents). Otherwise (N), the audio signal is not (significantly)adjusted.

After the low-frequency excursion protection, the method 700 applies 730a transient excursion protection. The transient excursion protectionincludes determining an instantaneous excursion prediction, which cancorrespond to an amount of excursion of the speaker based on a currentsample and previous sample(s) of the audio signal. The instantaneousexcursion prediction is then compared 732 to the maximum excursion. Whenthe prediction exceeds 733 (Y) the maximum excursion (or otherwisesatisfies an excursion condition), the audio signal is adjusted (e.g.,attenuated, compressed). Otherwise (N), the audio signal is not(significantly) adjusted.

The disclosed approach and its implementations can advantageouslyprovide versatility to a selection of an amplifier for a speaker, orvice versa. For example, the disclosed approach can allow a poweramplifier (e.g., a Class-D amplifier) to be connected to a speakerwithout worry that the speaker cannot handle its full output power. Thisversatility added may eliminate the need to create multiple amplifierdesigns to drive a variety of different speakers.

In the specification and/or figures, typical embodiments have beendisclosed. The present disclosure is not limited to such exemplaryembodiments. The use of the term “and/or” includes any and allcombinations of one or more of the associated listed items. The figuresare schematic representations and so are not necessarily drawn to scale.Unless otherwise noted, specific terms have been used in a generic anddescriptive sense and not for purposes of limitation.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art. Methods and materials similar or equivalent to those describedherein can be used in the practice or testing of the present disclosure.As used in the specification, and in the appended claims, the singularforms “a,” “an,” “the” include plural referents unless the contextclearly dictates otherwise. The term “comprising” and variations thereofas used herein is used synonymously with the term “including” andvariations thereof and are open, non-limiting terms. The terms“optional” or “optionally” used herein mean that the subsequentlydescribed feature, event or circumstance may or may not occur, and thatthe description includes instances where said feature, event orcircumstance occurs and instances where it does not. Ranges may beexpressed herein as from “about” one particular value, and/or to “about”another particular value. When such a range is expressed, an aspectincludes from the one particular value and/or to the other particularvalue. Similarly, when values are expressed as approximations, by use ofthe antecedent “about,” it will be understood that the particular valueforms another aspect. It will be further understood that the endpointsof each of the ranges are significant both in relation to the otherendpoint, and independently of the other endpoint.

Some implementations may be implemented using various semiconductorprocessing and/or packaging techniques. Some implementations may beimplemented using various types of semiconductor processing techniquesassociated with semiconductor substrates including, but not limited to,for example, Silicon (Si), Gallium Arsenide (GaAs), Gallium Nitride(GaN), Silicon Carbide (SiC) and/or so forth.

While certain features of the described implementations have beenillustrated as described herein, many modifications, substitutions,changes and equivalents will now occur to those skilled in the art. Itis, therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the scope of theimplementations. It should be understood that they have been presentedby way of example only, not limitation, and various changes in form anddetails may be made. Any portion of the apparatus and/or methodsdescribed herein may be combined in any combination, except mutuallyexclusive combinations. The implementations described herein can includevarious combinations and/or sub-combinations of the functions,components and/or features of the different implementations described.

The invention claimed is:
 1. An apparatus, comprising: a processorconfigured by software instructions to perform a method including:receiving an audio signal; receiving information corresponding to aspeaker; applying a low-frequency excursion protection to the audiosignal, the low-frequency excursion protection including a sub-resonanceexcursion prediction of a movement of the speaker in response to theaudio signal based on the information corresponding to the speaker;applying a transient excursion protection to the audio signal, thetransient excursion protection including an instantaneous prediction ofthe movement of the speaker in response to the audio signal based on theinformation corresponding to the speaker; and outputting a protectedaudio signal including the low-frequency excursion protection and thetransient excursion protection.
 2. The apparatus according to claim 1,further including a digital to analog converter configured to receivethe protected audio signal from the processor and to output a protectedanalog signal.
 3. The apparatus according to claim 2, further includinga power amplifier configured to amplify the protected analog signal andto output an amplified protected signal to the speaker, the amplifiedprotected signal having an amplitude below a level that over-drives thespeaker.
 4. The apparatus according to claim 3, wherein the poweramplifier is a Class-D amplifier capable of outputting an amplitude thatover-drives the speaker, the Class-D amplifier prevented from outputtingthe amplitude that over-drives the speaker by the low-frequencyexcursion protection and the transient excursion protection.
 5. Theapparatus according to claim 1, wherein the information corresponding tothe speaker is received from a sensor proximate to, or integrated with,the speaker.
 6. The apparatus according to claim 1, wherein theinformation corresponding to the speaker includes a position of thespeaker or a movement of the speaker.
 7. The apparatus according toclaim 1, wherein the information corresponding to the speaker includes adriving voltage or a driving current of the speaker.
 8. The apparatusaccording to claim 1, wherein the processor is further configured toderive speaker parameters from the information corresponding to thespeaker.
 9. The apparatus according to claim 8, wherein the derivedspeaker parameters are used to create or update a speaker model thatdescribes operation of the speaker, the sub-resonance excursionprediction and the instantaneous prediction of the movement of thespeaker based on the speaker model.
 10. The apparatus according to claim1, wherein the apparatus is a smart amplifier and the processor is adigital signal processor.
 11. The apparatus according to claim 1,wherein the applying a low-frequency excursion protection to the audiosignal includes: filtering the audio signal to obtain frequencies belowa resonant frequency of the speaker; determining, based on thefrequencies below the resonant frequency, that the sub-resonanceexcursion prediction exceeds a maximum excursion; raising a cut-offfrequency of an adjustable high-pass filter in proportion to an amountthat the sub-resonance excursion prediction exceeds the maximumexcursion; and filtering the audio signal using the adjustable high-passfilter to block frequencies of the audio signal below the raised cut-offfrequency to prevent an over excursion of the speaker caused by lowfrequencies.
 12. The apparatus according to claim 11, wherein themaximum excursion is adjustable.
 13. The apparatus according to claim 1,wherein the applying a transient excursion protection to the audiosignal includes: determining, from the instantaneous prediction of themovement of the speaker in response to the audio signal that a transientaudio signal is present; and attenuating a delayed version of the audiosignal to prevent an over excursion of the speaker caused by thetransient audio signal.
 14. The apparatus according to claim 13, whereinthe attenuating a delayed version of the audio signal to reduce aspeaker excursion caused by the transient audio signal includes:reducing a gain of an amplifier while the audio signal is delayed sothat the transient audio signal is amplified by the amplifier with thereduced gain.
 15. The apparatus according to claim 14, wherein thetransient excursion protection is applied after applying thelow-frequency excursion protection in order to reduce a delay used forthe transient excursion protection.
 16. A method to prevent overdrivinga speaker, the method including: receiving an audio signal intended forplayback on a speaker; receiving information corresponding to thespeaker; applying a low-frequency excursion protection to the audiosignal, the low-frequency excursion protection including a sub-resonanceexcursion prediction of a movement of the speaker in response to theaudio signal based on the information corresponding to the speaker;applying a transient excursion protection to the audio signal, thetransient excursion protection including an instantaneous prediction ofthe movement of the speaker in response to the audio signal based on theinformation corresponding to the speaker; and outputting a protectedaudio signal to the speaker, the protected audio signal including thelow-frequency excursion protection and the transient excursionprotection.
 17. The method according to claim 16, wherein the applying alow-frequency excursion protection to the audio signal includes:filtering the audio signal to obtain frequencies below a resonantfrequency of the speaker; determining, based on the frequencies belowthe resonant frequency, that the sub-resonance excursion predictionexceeds a maximum excursion; raising a cut-off frequency of anadjustable high-pass filter in proportion to an amount that thesub-resonance excursion prediction exceeds a maximum excursion; andfiltering the audio signal using the adjustable high-pass filter toblock frequencies of the audio signal below the raised cut-off frequencyto prevent overdriving the speaker.
 18. The method according to claim16, wherein the applying a transient excursion protection to the audiosignal includes: determining from the instantaneous prediction of themovement of the speaker in response to the audio signal that a transientaudio signal is present; and attenuating a delayed version of the audiosignal to prevent overdriving the speaker.
 19. The method according toclaim 18, wherein the attenuating a delayed version of the audio signalto reduce a speaker excursion caused by the transient audio signalincludes: reducing a gain of an amplifier while the audio signal isdelayed so that a transient audio signal is amplified by the amplifierwith the reduced gain.
 20. The method according to claim 19, wherein thetransient excursion protection is applied after applying thelow-frequency excursion protection in order to reduce a delay used forthe transient excursion protection.